Blister packs are well known and widely used for packaging of various commercial products such as tools, batteries, toys, and products from pharmaceutical markets. The packages are commonly referred to as “visual packaging” where at least one side of the package is made of a transparent and formed plastic material (a “blister”) to house the packaged articles and thereby allowing the customer to see the products inside the package. The blister is commonly bonded to a sheet-like substrate (backing) coated with a heat seal coating to enclose the packaged articles in a separate step. Paperboard, when employed as the backing material and carrier of printed information, is referred to as blister board. US 2006/0246262 A1, US 2003/0148110 A1, and US2003/0196925A1 describe requirements for, and improvements to, blister boards. The heat seal coatings are applied by roller coaters or flexo coaters from flexo or offset presses onto the sheet-like substrate in sheet-fed or roll-fed operations. In the heat sealing step, the heat seal coated backing and the plastic blister are pressed together by means of a heated sealing jar.
Since the application of the heat seal coatings and the bonding of the blister are carried out in different operations, a heat seal coating must in some cases meet contradictory requirements as outlined below.
The printed and coated substrates or paperboards (blister cards) must be able to be stacked without sticking together (blocking) during stacking, storage, die-cutting, and transportation in temperatures from ambient to 50° C. Block resistance up to 50° C. under pressure is essential so that the printed and coated sheets can be transported to and processed at the packing and sealing line. During the sealing process, the coating will be activated to behave like an adhesive at an elevated temperature and bond to the blister plastic. It is important that the activation temperature be not high enough to melt the blister plastic. Thus, the desired activation temperature is typically in the range of 170° F. to 240° F. (76° C. to 116° C.). Because of the dual function of coating and having adhesive properties at different temperature ranges, the heat seal compositions are referred to either as heat seal coatings or as heat seal adhesives. The term “heat seal coatings” shall be used herein to describe both heat seal coatings and heat seal adhesives. It is of great importance that the blister packs also provide visual evidence (such as fiber tearing of the paperboard backing) of any unauthorized or unintentional entry into the blister pack, and that they do not rupture under conditions such as shipping, handling or stocking or being dropped on the floor. The heat seal coatings have to provide a strong enough bond to provide for such substrate tearing, such as fiber tearing for a paper board backing, and must be resistant to high moisture levels that may be present during transportation, storage, and store handling periods.
Due to its negative impact on environment, PVC, once the primary plastic for blister use, is intentionally avoided nowadays. Virgin PET, APET, recycled PET (RPET), PETG, and GAG have become the blister materials of choice for many brand owners. However, adhesion of the heat seal coating to these new plastics is difficult, especially for waterborne and solvent borne heat seal coatings alike. For environmental sustainability, the use of a recycled material like RPET is desired, but such materials are especially challenging to achieve good fiber tearing seal performance.
Solvent based blister coatings have been used for blister packs for many decades and still are being used by small number of converters, but it is desirable to provide a functional waterborne heat seal coatings for environmental reasons. Many converters use offset presses to apply both inks and coatings in-line, where there is only a moderate drying capability. Waterborne heat seal coatings have to dry quickly on such lines without reducing press speed, and must provide good press runnability and ease of cleaning. In addition, waterborne heat seal coatings have to meet all the requirements mentioned above.
Currently there are two aqueous chemistries being used in the marketplace for blister heat seal applications, namely polyurethane dispersions and acrylic emulsion/dispersions. Polyurethane dispersion systems, in general, may have good heat seal adhesion to various blister plastic mentioned above. However, they suffer from major drawbacks of having a tacky coated surface, requiring a slower line speed, having extensive/difficult cleanup procedures, and having a high cost. The convertors have employed many tactics to address the blocking related issues, such as applying offset (starch) powders, stacking in short piles, and packing vertically to minimize the stacking pressure. Aggressive solvents and frequent ultrasonic deep cleaning for anilox rollers have to be employed for cleaning and maintaining consistent coat weight. Due to the roller clogging and surface tack issues their press runnability suffers.
Acrylic systems, on the other hand, have good press runnability but have very limited heal seal adhesion to non-PVC blisters, especially virgin PET, APET, and RPET, causing many package failures during transportation, cold storage conditions, and in the stores.
U.S. Pat. No. 5,385,967 and EP 0798357B1 disclose aqueous dispersions for heat seal applications with a first copolymer having a glass transition (Tg) of 50 to 150° C. and a second copolymer with a glass transition of −50 to 50° C. with the Tg of the two polymers differing by 20° C.; at least one of the copolymers contains 3 to 70% by weight of an acrylic ester unsaturated C3-C5 mono- or di-carboxylic acid or anhydrides thereof. The recited examples generally show a high glass transition temperature (Tg) copolymer with high amount of amount of C3-C5 mono- or di-carboxylic acid or anhydrides thereof. Such high Tg polymers with a high amount of carboxylic resins are generally known as support resins and, while having good colloidal stability, tend to impart unacceptable moisture sensitivity in applications where exposure to moisture occurs. Both aqueous copolymers are ethylenic copolymers and do not contain any polyurethane dispersion or wax, the block resistance is only presented at 40° C., and the heat seal is only shown with PVC, PS (polystyrene), PET and not the more difficult RPET blister.
U.S. Pat. No. 5,800,873 relates to a process for producing sealed packaging containers from plastic films, comprising the step of applying to the plastic film a sealing coating based on an aqueous dispersion or solution of acrylate copolymer which is prepared by emulsion polymerization of alkyl ester of acrylic acid and methacrylic acid and α,β-ethylenically unsaturated carboxylic acids, wherein the aqueous dispersion or solution is prepared by a 2-stage emulsion polymerization. The Tg of the copolymer is not more than 50° C. The patent further describes that the film has to be pretreated with an oxidizing agent (such as chromic acid or a chlorination, hot air, steam, or flame or corona treatment) or pre-coated with a primer (such as alkyl titanates and polyethyleneimines) to ensure adhesion. The sealing temperature is in the range of 20° to 80° C., and particularly 40° to 80° C., which is too close to the 50° C. block resistance temperature required for blister heat seal.
WO 2011/017388A2 relates to a heat seal adhesive composition for plastic and metal containers, and also includes only ethylenic copolymers; 50 to 75% by weight of soft acrylate copolymers having a Tg from 0 to 60° C., containing only 0.2-10 weight % of C3-C10 ethylenic unsaturated mono- or dicarboxylic acids or its anhydrides and 25-50% by weight of hard acrylate copolymers having a Tg from 50 to 120° C., containing up to 10% of C3-C10 ethylenic unsaturated mono- or dicarboxylic acids or their anhydrides. This approach is similar to that disclosed in WO 2011/017388A2 mentioned above, only with a less acidic moiety. It utilizes a hard acrylic copolymer to improve block resistance, and does not contain polyurethane dispersion or wax. The heat seal activation temperature is 120 to 175° C., which is close to the softening or melting point of some of the plastics in considerations, resulting in a long heat seal time; a heat seal test is disclosed as only conducted with adhesive coated aluminized polyester and polystyrene (PS); adhesion to recycled PET is not mentioned.
WO 2001077246A1 discloses a low temperature thermally activated water dispersed adhesive compositions made from low modulus crystallizing polyester polyurethane, acrylic ester copolymer, one or more hydrophobic modified associated polyurethanes, and a stabilizer comprising a combination of carbodiimide and a branched primary amino alcohol. Even though the composition is heat sealable and defines its adhesive behavior with a modulus from dynamic mechanical properties, it has an open time limitation, beyond which it is not heat sealable. It is designed specifically for bonding fabric to foam for automotive and office seating; no blister plastics as mentioned above are identified; the bond line temperature for the heat seal is at 43° C., which would create a blocking issue in blister applications. The stability of the blend has to be stabilized by an associative thickener and a combination of carbodiimides and a branched primary amino alcohol.
U.S. Pat. No. 6,924,366 B2 teaches a polymer dispersion for heat seal lamination useful for glass fiber sizing, comprising a water continuous phase, dispersed particles of polyurethane, (non-aromatic, with a melting point of 25 to 70° C.); and a copolymer and/or terpolymers having a Tg between −20 and 50° C., emulsion polymerized in the presence of a polyurethane dispersion (termed a “Hybrid” polymer blend). It is used as heat seal lamination adhesive between MDF wood and PVC, and requires 1 hour drying at room temperature, which is too long for a blister coating application. The reported blocking test was conducted by a finger tack without any pressure, which is not sufficient for blister application where coated sheets are stack in a pile under high pressure. The bond performance also requires 24 hours to develop, which is not practical for blister packaging processes.
U.S. Pat. No. 8,653,180B2 provides a composition with early hardness development comprising a blend of one or more emulsion vinyl copolymers having a Tg of 20° C. or higher, one or more polyurethane copolymers in aqueous dispersion, and one or more additives chosen from coalescing solvents with boiling points from 150 to 300° C. and naturally derived plasticizers. The composition enhances hardness development and reduces VOC (volatile organic compounds) compared to solvent based systems, but, a high boiling point solvent required for fast hardness development will need extreme heat for its removal, and no heat sealability is mentioned.
A dry blond laminating adhesive is mentioned in U.S. Pat. No. 6,248,815 B1 to offer improved bonded properties of flexible film substrates such as polyolefins, polyesters, polyamides, cellophanes, metals and papers. The composition comprises at least one aqueous vinyl polymer dispersion, at least one water dispersible polyfunctional aziridine, and optionally at least one polyurethane polymer dispersion. The adhesive does requires 3 days to cure to the bond strength, which is not desirable for blister application with an immediate fiber tearing bond requirement, and it contains a hazardous aziridine crosslinker.
U.S. Pat. No. 8,637,609 B1 relates to blends of 40-60 wt % of acrylic silane polymers and 60-40 wt % of a polyurethane/acrylic hybrid dispersion for use on a plastic or metal substrate. However, there is no specific recitation of heat seal properties, and no other specific application is mentioned. CA 2086209A1 utilizes a similar acrylic silane polymer and polyurethane for automotive finishes and is not heat sealable.
WO1994005738A1, EP 0656926 B1 and CA2140938C provide a book binding primer and adhesive composition comprising a) an aqueous vehicle, b) 1-90% of a polymeric plastic film-forming resin comprising an ethylene-vinyl acetate copolymer, a styrene butadiene rubber, an acrylic resin, an acetate butyl maleate copolymer or a mixture thereof, c) 10-99% of a polyurethane resin comprising a hindered aliphatic or aromatic-aliphatic polyurethane resin and crosslinkable. The booking binding adhesive composition provides use as primer, with asserted shape retention, excellent bond strength, and acceptable drying characteristics due to the blend. However, there is no mention of heat sealability, plastic adhesion or use in blister applications.
U.S. Pat. No. 5,548,016 teaches a primer composition for a flexible plastic substrate that includes a blend of 10-30% resin (80-90 wt % of acrylic and 10-20 wt % of urethane resins), 20-30% pigments, 10-25% water dispersible fast drying coalescing solvent, and water. The resin blend provides adhesion to thermal-plastic olefin (TPO) and sheet molded compounds (SMC). No heat seal property is mentioned and the fast drying properties is based on high percentage of fast drying coalescing solvents in the system. U.S. Pat. No. 5,006,413 provides an asserted good water borne alternative to solvent based systems for after-market finishing repair, and features use of an acrylamide-acrylic copolymer with polyurethane or polyacrylo-urethane. It is designed for coating only and not for heat sealing. US 2015/0045491 A1 and WO2013139019A1 utilize polyurethane/acrylic hybrid dispersions (“chemical” blends) containing acidic monomers to achieve a high pigment volume concentration (PVC) roof coating (high binder affinity) and a lower raw material cost. CN103360561 B discloses a hybrid preparation method to make a polyurethane modified acrylic emulsion to improve the compatibility between the originally two incompatible polymers to form an interpenetrating network. This chemical blending is claimed to provide better transparency, molecular mixability, hardness, and other superior properties for artificial stones. No heat seal property is mentioned.
U.S. Pat. No. 4,654,397 teaches the preparation of an acrylic copolymer dispersion with low film formation temperature but still yielding films having good block resistance. The dispersion is prepared through a multistage polymerization process, and comprises A) 75-98% of a monomer yielding a homopolymer with a Tg from −72 to 0° C., B) 2-25% of a monomer yielding a homopolymer with a Tg 80 to 140° C., C) 0-10% carboxyl or carboxamide monomer, and D) a monomer with an effect on crosslinking. The anti-blocking property comes from controlling the amounts of high Tg monomer, acidic monomer and crosslinking monomer without using wax, while the soft monomer provide the low temperature film forming characteristics. However, specific adhesion to plastics and heat sealability is not mentioned.
U.S. Pat. No. 5,756,170 relates to a packaging film coated with polyurethane which bears hydrophilic groups which makes it water dispersible and has a melting from 20 to 70° C. with an enthalpy of fusion at least 20 J/g. The specified composition which was tested for its blocking resistance at ambient temperature and humidity only, is asserted to seal well to oriented polypropylene (OPP) film at 50° C., which is the block resistant temperature for blister applications, and no other plastics are mentioned. It also requires a 2 part polyurethane primer on OPP. The specific components (isocyanates and polyols) used to prepared the polyurethane dispersion are too soft and can only be used for cold seal adhesive type applications.
None of the above mentioned prior art address all the market requirements for blister heal seal coatings, which include the ability to heat seal to various blister plastics (RPET, APET, virgin PET, PETG, GAG, PVC), tack free coating surface off the press, good press runnability with easy clean-up, good block resistance, a proper heat activation temperature per blister coating applications, and low VOC contents. There remains a need in the marketplace for a waterborne/aqueous heat seal coating that meets all the requirements mentioned above.